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Growth of bacterial population in dynamic and spatially extended environments


Growth and reproduction are essential requirements of living systems. Previous studies of bacterial physiology focused largely on bacteria growing exponentially in fixed environments. But such growth conditions are exception rather than the norm in nature. Bacteria in the wild must cope with environments that vary temporally and spatially. To gain better understanding of bacterial physiology, I studied growth of E. coli population in 1) dynamic nutrient environment and 2) spatially extended environment.

Rapid adaptation is a crucial determinant of bacterial fitness under changing environments. We studied adaptation of E. coli culture to abrupt change of nutrient from preferred sugars (e.g., glucose) to a metabolic waste product (e.g., acetate) that is excreted during the growth on preferred substrates. In such transitions, E. coli suffers long, multi-hour periods of growth arrest before resuming growth. By characterizing the period of growth arrest (lag time) for many pairs of carbon sources, we discovered that cells growing fast before the shift exhibit longer lag time, resulting in a remarkable trade- off relationship between fast growth and adaptability. We establish that this tradeoff results from switching the direction of central carbon metabolism from glycolysis to gluconeogenesis. The existence of tradeoff limits fitness benefit gained from fast growth and provides a criterion for selecting growth rates based on the expected abundances of substrates in the environment.

In spatially open habitats, effective colonization provides a selective advantage for bacterial populations. We next studied the growth of E. coli population that expands by chemotaxis in nutrient-replete soft-agar plates. We show the speed of population expansion, as exemplified by the propagation of chemotactic rings, increases steadily with improved growth conditions. The expansion process is shown to couple intricately with colonization, with the propagating cells leave behind a trail of offspring that slowly proliferate over time. We also show costly expression of motility apparatus is necessary to maintain the fraction of motile cells and support faster population expansion. The results provide a link from gene expression through cellular motility all the way to population dynamics, elucidating a foresighted strategy bacteria use to colonize virgin territories well before local nutrient supply runs out.

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